Many years ago most furnaces and boilers were fired by coal. More recently, as natural gas and oil became more convenient and less expensive, and as home owners and industrial and commercial plant operators realized that gas and oil were cleaner than coal, furnaces and boilers fired by natural gas and oil became more prevalent with the result that up until the last several years coal-fired furnaces and boilers have become more or less obsolete.
Recently, with all the concerns about availability of gas and oil, the increase in price of gas and oil, and the possible and potential administrative restraints over the use of gas and oil, coal has become much more feasible as an alternative fuel for furnaces and boilers. Coal costs are now competitive or perhaps even less expensive than natural gas and oil. Therefore, if the other disadvantages of coal such as convenience, efficiency, and cleanliness can be overcome, coal might become much more attractive as a fuel.
Normally, furnaces or boilers which are fired by coal include a stoker which feeds coal into a box where the coal particles are distributed onto a hot bed of burning coals, rather than feeding the coal through a burner nozzle as is the case with natural gas and oil. Compared to natural gas and oil this is an inefficient burning process because a relatively low percentage of the potential combustible portion of the coal burns, leaving many ashes and clinkers, and also resulting in much smoke and soot because of the relatively low heat effected by a bed of burning coals compared to a burner nozzle.
On the other hand, the relatively high temperature of combustion required to ignite coal makes it hard to burn as it exits rapidly from a nozzle as compared with natural gas and oil. In other words, coal will not ignite as rapidly as other fuels when it exits from a nozzle, because the high temperature of combustion is hard to obtain in a relatively short amount of time by an ignition system. It is known that it is necessary to provide an igniter of either oil or natural gas which provides an ignition temperature in the range of 3500.degree. to 5200.degree. F. in order to get the previously known pulverized coal particles ignited and burning. Such attempts have utilized coal as the primary fuel to be burned in nozzles by reducing the particle size somewhat and igniting a mixture of coal particles and air as the coal particles exit from the nozzle as disclosed in the Graybill U.S. Pat. No. 4,147,116. Utilizing a coal particle size of 40 microns, the particles are maintained in an inner gaseous environment to prevent spontaneous combustion, then delivered through a nozzle which separates the inner gas from the combustible coal particles, whereupon the coal particles are mixed with air which will support combustion and deliver it into a combustion zone past an oil or natural gas igniter. Other attempts have utilized electric arc heated, high velocity, oxidizing gas jets as the igniter for pulverized coal dust. While these attempts may or may not have been successful, the use of secondary petroleum base fuels or other igniter systems are expensive and generally require the use of oil or gas in addition to the coal.
A further disadvantage of coal as a fuel, as set forth hereinabove, is the exhaust gases of combustion resulting from coal are relatively dirty and difficult to clean. This is true because many coal particles and particles of contaminants (fly ash) in coal do not burn with the result that there are so many particles, and some of them fairly large, in exhaust gases that normal filter techniques quickly clog up making the filtration thereof almost impossible. Also, the sulfur gases and other gases remaining in the exhaust fumes of burning coal particles require more than mechanical filters to remove.
Introducing air or other gases which support the combustion of the coal particles near the nozzle where they are mixed with the coal particles does not result in a good homogenous mixture of the coal particles and air at the combustion zone. Further, control of the mixture being provided to the burner is difficult when the air is coming from one point and coal from another. Finally, as is true in all pulverized coal feeding systems, an airborne feed system is difficult to maintain operational because the very small coal particles tend to clog feed lines as coal particles of this size are relatively tacky to the material making up the feed lines.